Short answer:

Choose one to be the master catalog and merge the other two using the Import from Another Catalog feature.

Or you could create a new blank catalog and merge them all in using the Import from another Catalog feature if you want to avoid any clutter in your existing catalogs.

You can find edited photos using a smart collection. So open a catalogue and create a smart collection biased on Has Adjustments and/or Edit Time. This will detect edited photos and allow you to find which ones should go into the new master catalogue.  When sure, do an Export as Catalog for these items and import it into the new master. Repeat until done. So essentially I'm saying sort out in each catalogue which images should be in the final one.

Alternatively, decide which catalogue is the LEAST  up-to-date, and just import the other catalogues into it. This combined catalogue will now contain the  updated versions of the images. Keep the two old catalogues , just in case.

And backup everything before doing anything.

When you import one catalog into another, Lightroom will identify duplicate files and give you options as to how you want those duplicates handled. One of the options allows you to import the dupes (i.e. the same file, not a second copy) but create a virtual copy of the originals so that you can later compare the two sets of edits and decide which one you want to keep.

Possibly, John. I guess a lot would depend upon the extent of the duplication, and more importantly how many have duplicate (different) edits in the two catalogs. IIRC, importing an unedited file into a catalog which already has that file will result in the unedited file being ignored. Doing all the sorting out prior to running the catalog import as per your suggestion would eliminate much confusion. However, the point is that when running the "Import from Another Catalog" that "Changed Existing Photos" box WILL be reported, so it would be as well for the user to be aware....of course  "Don't Import New Photos" is one of the options, which I guess would be your recommendation provided it is certain that the "older" edits are in the catlog being imported.

It doesn't matter what the name is.

If you Export As Catalog, then its a catalog not a smart collection.